[hpsdr] ALEX - Input protection

[Graham Haddock]

Henry:
Comments embedded below.

Henry Vredegoor wrote:
> Would a (broadband) input transformer help in this respect?
>   
It depends.  Most of the energy generated by a lightning strike is between
5 kHz and 300 kHz.  Only a very small percentage is above 1 MHz.  So, if the
bandwidth of the "broadband" transformer had a low frequency cut-off
of around 160 meters, and the transformer provided a very low
impedance to ground below 500 kHz, then it could help a lot.  If it was
truly broadband, passing energy down to the tens of kHz, then it would not
help much at all, other than to provide a DC path to ground.

In most ham radios, it is the narrow per-band bandpass filters that
limits the energy getting to the front end of the radio, not really the
voltage limiter at the input.

The goal of MERCURY seems to be to provide a very broadband receiver,
enabling all kinds of experiments, test equipment operation and
multi-channel and multi-band receive, which is somewhat at odds
with the transient protection.
> A permanent connection to ground of one side of the 50 Ohm input winding
> would discharge the antenna system permanently.
>   
ALEX will have some 10K resistors to ground to bleed off any DC static
build up.
> Also as ground loop prevention?
> Could be part of some filter in ALEX ? 
>   
All of the filters are unbalanced, with one-side hard grounded.
> IP3 of the device could be a problem.
>   
Surprisingly, the IP3 performance of the ferrite input/output
transformers that Phil and I looked at for some alternative
ALEX designs had extremely good IP3 performance,
that is, above +50 dBm.  This surprised me in light of the
way that the powdered iron toroid inductors do limit
IP3.


So, I will have a bidirectional voltage clamp on each ALEX receiver
input that will start clamping around 3 volts, and still clamp
below 20 volts, even in the presence of tens of amperes of
transient.  This is not lightning protection, you still need to
shunt the lightning energy to ground before it ever gets to
your equipment.  This is transient protection to limit the
input voltage to sensitive semiconductor switches, attenuator,
and amplifiers, as best we can.

--- Graham / KE9H

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